The long-term objective of this research is to determine how ovarian steroids trigger luteinizing hormone-releasing hormone (LHRH) and LH surge release, thereby inducing ovulation. In these studies, we will investigate mechanisms through which potoperiod and estrogen signals interact to regulate novel dual-phenotype GABA/glutamate neurons in the anteroventral periventricular nucleus (AVPV) of females. The specific aims of this proposal are: 1) To verify that GABA/glutamate neurons communicate directly with LHR neurons we will use anterograde and retrograde tracing and immunocytochemistry in rats and mice;2) We will use electrophysiological studies in which LHRH neurons are identified by GGP expression to verify that AVPV GABA/Glutamate neurons release predominantly GABA during the morning and predominantly glutamate in the afternoon;3) To determine whether vasopressin (VP) and/or noradrenaline (NA) trigger the midday rise in GABA release linked to the LH surge. We will examine the effects of specific VP and NA receptor antagonists on expression of a marker of GABA release, glutamic acid decarboxylase 67 (GAD67) mRNA. We will also determine whether changes in GAD67 mRNA levels are blocked when animals are placed in constant light and if such changes, as well as LH surge release, can be induced with VP and/or NA receptor agonists;4) To determine whether autofeedback inhibits GABA synthesis while stimulating glutamate release prior to the LH surge, we will administer GABAB receptor antagonists and measure changes in GAD67 mRNA levels in AVPV cell bodies, as well as vesicular GABA transporter and vesicular glutamate transporter in dual-phenotype terminals contacting LHRH neurons;5) We will determine whether VIP stimulates cAMP-dependent NT gene expression through ligand-independent activation of PR on the afternoon of the LH surge. The results of these studies may provide new targets for examination in humans and nonhuman primates and may lead to a better understanding of the neural aspects of puberty, infertility and hypothalamic aging. In addition, establishing that GABA neurons are able to switch from inhibitory neurotransmission to excitatory transmission will have important implications for understanding GABA and glutamatergic "dual-signaling" in other brain regions. Finally, the results of these studies may provide key insights into how E2 regulates such diverse functions as sex-specific neurodevelopment, neuroprotection and neurodegeneration